According to our database there was a known issue with the exact same error message in the previous build version but it was fixed: https://kb.acronis.com/content/48026
The root cause was the faulty SnapAPI module. I suggest eliminating this root cause in your case by installing the latest SnapAPI module for Acronis Backup 11.7.

Use the following link to download msi file for installation: https://access.acronis.com/t/6jr7x51h

I would also like to recommend you to restart Volume Shadow Copy Service. Also make sure that "Software - System provider" is chosen in your backup plan under "Volume Shadow Copy Service": https://www.acronis.com/en-us/support/documentation/AcronisBackup_11.7/#12865.html